Packings or seals include those means used to prevent or minimize leakage of a fluid through mechanical clearances in either the static or dynamic state. Dynamic seals include all packings that operate on moving surfaces. In functioning to retain fluid under pressure, the dynamic seal carries the hydraulic load. When no pressure exists, the packing is mechanically loaded as by a spring or by its own resiliency. Dynamic packings therefore operate as bearings, thereby indicating the need for lubrication to serve as both a separating film and a coolant. The presence of a film is vital for satisfactory service life for both the seal and the shaft. However, the film also leads to leakage across the seal.
Dynamic seals are generally classified in three ways. First, they are classified on the basis of the shape of the surfaces: cylindrical, conical, spherical or flat. Cylindrical packings are in turn classified according to whether they pack on the outer perimeter, as in piston packings, or the inside perimeter, as on rods or shafts. Second, seals are classified on the type of motion involved; rotary, oscillating, reciprocating, or helical. Last, seals are classified on the basis of their (non) performance: automatic or nonautomatic. For example, soft or jamb packings are tightened by external means, generally a gland, while automatic-performed seals are self-tightening under pressure.
An example of a pressure balanced circumferential seal assembly is provided by U.S. Pat. No. 3,575,424 to Taschenberg. This Taschenberg patent discloses a segmental circumferential shaft seal purported to provide both axial and radial balance by using the distribution of fluid pressure forces applied to opposing surfaces of the seal. The Taschenberg seal also uses radial forces acting on the end gaps between the sealing ring segments, which would otherwise tend to unseat the sealing ring, thus causing excessive leakage across the seal. Taschenberg balances the resultant forces to permit the axial and radial seating forces to slightly exceed the axial and radial lifting forces, thus creating a seating bias. A second seal ring or backup ring in the seal assembly forms a second sealing interface with the segmental primary seal ring. A bellows diaphragm connects the secondary seal with a second shoulder of the seal assembly adjacent the high pressure side, thereby preventing the passage of high pressure fluid to an area surrounding the outer circumference of the primary and secondary seals. Passages within the seal assembly permit the flow of high pressure fluid or the flow of low pressure fluid to achieve pressure balancing. Thus, Taschenberg uses two secondary seals, one of which is a bellows which limits pressure capability. Furthermore, Taschenberg relies on rubbing contact rather than a fluid film to achieve a seal between the primary seal element and the rotating shaft.
A need exists for a circumferential sector seal with high pressure capability. Such a seal should use fluid-film lubrication between the seal and the rotating shaft. Individual sector movement should allow for shaft vibrations, and centrifugal and thermal shaft excursions without contact or wear. The sector seal should be bidirectional in operation and allow unlimited axial movement of the shaft. The sector seal should also be capable of taking the entire pressure differential across a single axial length.